Continuous curve countersink and rivet head structure for riveted work pieces

ABSTRACT

A fastening system having a continuously curved countersink section in an aperture through a plurality of workpieces, with a corresponding rivet having a continuously curved head portion substantially congruent to the continuously curved countersink. Rivets equipped with the continuously curved head surfaces taught herein, when mated with a continuously curved countersink, provide sufficient resistance to fastener pull through, cause cold working of the rivet upon insertion of the rivet into the aperture and impose an elastic load upon the workpiece and rivet, strengthening the joint.

This is a divisional of application Ser. No. 865,933, filed Dec. 30,1977 now U.S. Pat. No. 4,245,921, issued Jan. 20, 1981, which is acontinuation-in-part of application Ser. No. 755,834, filed Dec. 30,1976, now U.S. Pat. No. 4,111,580, issued Sept. 5, 1978.

BACKGROUND OF THE INVENTION

This invention relates to means for fastening workpieces togetherutilizing rivets with formable ends having continuously curved headportions insertable into bores having substantially congruentcontinuously curved countersinks.

PRIOR ART

In the fastening of metal workpieces together with rivets, bolts and pinfasteners, the necessity of interference in the countersink area hasbeen demonstrated by extensive testing and experience in the aerospaceindustry. Countersink structures having diverging angles of 82° and 100°have been found to be associated with numerous fatigue failures,originating in the countersinks. The geometry of these countersinkstructures is such that the fastener inserted therein cannot impart asufficient wedging force (interference) to the material at thecountersink. As a result, a condition is present that allowsinfiltration of moisture and foreign materials between the rivet and thecountersink. Corrosion results. Efforts have been made to solve theproblem of corrosion by the addition of a thick paint coating to isolatethe exterior surface of the workpieces from the elements. However, theprotection is lost if a break occurs in the paint near the countersinkarea, allowing infiltration of moisture and foreign materials. Since thetypical 82° and 100° fasteners frequently permit working between thematerial and the fastener, such breaks readily occur. Furthermore, theapplication of protective paint surfaces adds substantially to thedeadweight of an airplane or other structure, resulting in loss ofperformance and high fuel consumption. In the instance of an aircraft,the structures which utilize rivets, bolts and pin-fasteners areconstantly under the cyclic influence of temperature, atmosphericpressure, dynamic stresses and moisture. As a result, a crack in thepaint can initiate a corrosion nucleus. From the initiation of thisnucleus and upon continued cyclic influences as mentioned, corrosionwhich may be visually concealed can spread to large areas, both at theinterface of the paint and the fasteners and at the countersinkboundary. Catastrophic failure due to stress corrosion may occur.Countersinks having a discontinuous surface also present location ofstress concentration and tend to harbor foreign matter.

The high fatigue squeeze riveting process shown in my U.S. Pat. No.3,874,070 aids in resolving certain of the shortcomings of the prior artrivets. The disclosure of U.S. Pat. No. 3,874,070 is incorporated hereinby reference as if fully set forth. While improved rivet performance andjoint strength is obtained by my prior squeeze riveting process, thereremains the problem of stress concentrations at the discontinuouslyangled zones of the countersink and uneven cold working of the rivetacross the head thereof with resultant uneven hoop stresses imposed uponvarious zones of the countersink. The continuously curved countersinkand rivet structure taught herein is ideally suited for use in theprocess of U.S. Pat. No. 3,874,070 as well as other conventionalriveting processes.

OBJECTS OF THE INVENTION

It is one object of this invention to provide a continuously curvedcountersink structure and a continuously curved rivet head structurewhich obtains a sufficient working of the rivet head material within thecountersink to obtain structural integrity, yet provides resistance topull-through of the rivet when installed.

An additional object of this invention is to provide a fastening systemfor fastening workpieces together which uses a continuously curvedinterface surface between the rivet and the countersink which preventsstress concentrations and avoids inclusion of foreign matter between thecountersink and the rivet.

It is a further object of this invention to provide a rivet fastenersystem which achieves a selectively controlled interference fit with allor part of the areas of a countersink and bore in the fastening system.

It is a further object of this invention to provide a fastening systemhaving superior corrosion and stress resistance.

It is a still further object of this invention to provide a fasteningsystem comprising workpieces and a rivet which prevents stress corrosionin the assembled structure by achieving a controlled laminarinterference pattern in the material surrounding the countersink and inthe rivet itself.

It is a specific object of this invention to provide a continuouslycurved countersink and a corresponding, substantially congruent, rivetfastener structure which is inexpensive to manufacture and which obtainssuperior corrosion resistance and fatigue life in the assembled parts.

SUMMARY OF THE INVENTION

A fastening system is disclosed comprising a workpiece having anaperture therethrough including a countersink having a continuous curvefrom the bore to the surface and a corresponding rivet having a headstructure which is continuously curved from the shank to the end of thehead. The countersink is defined by a surface of revolution, thegeneratrix of which comprises a continuously curved line extending fromthe bore of the workpiece, defining a continuous curve from the interiorof the workpiece to the surface thereof. One preferred form of thecountersink may be defined by rotation of a line comprising arcs of twocircles about a line substantially coincident with the axis of the bore,the first arc being tangent to the bore through the workpiece andpositioned interiorly of the edge thereof, and the second circle beingtangent to the first circle and intersecting the edge of the workpiece.Curves other than circles may be used. Other embodiments of thisinvention may be formed by having the line about which the generatrix isrotated being angled from or skewed with respect to the axis of thebore, so long as a continuous, smooth curve is attained from the bore tothe surface. Sections of parabolas, ellipses and other smooth curves maybe used as the generatrix without deviating from the applicant'sinvention. A corresponding smooth, curved structure is formed on theheads of the rivets utilized in conjunction with the continuouscountersink. A portion of the countersink near the bore which is flaredoutwardly from the axis of the bore provides resistance to pull-throughof the rivet. The portion of the countersink near the surface of theworkpiece is contoured more nearly parallel to the axis thereof so thatthe interference fit necessary to induce hoop stresses and improvestress corrosion resistance, as well as to obtain the desired seal, isobtained as the rivet head is deformed during assembly. Both elasticload on the workpiece and rivet and controlled laminar interferencepattern in the cold worked rivet material are obtainable in a properlydesigned fastener and rivet system.

While in most workpiece-rivet systems for forming riveted joints therivet is deformed and the workpiece substantially undeformed due to therelatively harder workpiece material, it is desirable in this inventionto utilize materials for the rivet and workpiece which permits a portionof the deformation necessary to join the workpieces by riveting to occurin the workpiece bore, that is, to have deformation both in the rivetand the workpiece. This advantageous arrangement causes cold workhardening of the metal in both the rivet and adjacent workpiece materialas the rivet is upset.

These and other aspects of this invention will be more readily apparentupon an examination of the description which follows and the appendeddrawings referred to therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partial side elevation of a rivet having the headstructure taught by this invention shown therein, positioned above andin exploded view with respect to an aperture in a workpiece having acontinuously curved countersink at one end of the aperture to receivethe rivet.

FIG. 2 shows a cross section of an assembled joint structure using thecontinuously curved countersink in conjunction with a rivet having acontinuously curved head structure. The rivet is shown in relief.

FIG. 3 shows a cross section of a joint structure wherein a rivet, inrelief, and countersink are shown formed with surfaces defined by ageneratrix comprising segments of two circles, the upper circle havingits center positioned below the workpiece surface.

FIG. 4 shows a device similar to that shown in FIG. 3 wherein the centerof the upper circle forming the generatrix for the countersink ispositioned above the surface of the workpiece.

FIG. 5 shows a cross-sectional view of a structural joint wherein thegeneratrix of the countersink is a section of an ellipse.

FIG. 6 shows a cross-section of a structural joint having the center ofthe upper portion of the generatrix for the countersink positioned atthe surface of the workpiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring specifically to FIG. 1, there is shown in an exploded view arivet fastener 20 having a head structure particularly adapted to beinserted within a continuously curved countersink formed in a workpieceand a workpiece 36 so fashioned with a continuously curved countersink28. The rivet 20 is comprised of a shank portion 22 and an enlarged headportion 24. Shank 22 may either be straight or tapered with respect tothe longitudinal axis thereof. An interference fit is preferably formedalong the length of bore portion 26 of the workpiece 18. In thepreferred embodiment shown in FIGS. 1 and 2, the countersink 28 is asurface described by the rotation of a curved generatrix about the axis30. In FIG. 1, the generatrix for the countersink 28 is formed ofsegments of two circles, the first circle 31 being a small diametercircle positioned at the base of countersink 28 having a radius R₄.Circle 31 is located within the workpiece 18 and is positioned tangentto bore 26 at point 27. The second segment of the generatrix is formedof a second circle having a radius R₂ measured from center 32 and whichis tangent to circle 31 at point 34. The countersink surface is definedby rotation of the above-described generatrix about axis 30. Theresultant countersink 28 is a smooth, curved surface extending from thebore 26 to the surface 36 of the workpiece 18.

The exterior surface of the head of rivet 20 is formed in a shapeslightly larger than and substantially congruent to the above-describedcountersink shape. The shape is defined by rotation of a generatrixformed of two curved segments, the first segment being circle 41, havinga radius R₃. Circle 41 is tangent to shank 22 at tangency point 40. Thecircle 41 also has point of tangency at 42 to a second, larger diametercircle 44 having a radius R₁. Circle 44 is the second curved segment ofthe generatrix. The fastener head is described by the surface generatedby rotation of the above-described generatrix about axis 30'. A line 45is drawn tangent to the point of tangency between circle 44 and circle41 lies at an angle A of approximately 41° with respect to the axis 30'.The line 46 drawn tangent to circle 44 at the intersection of circle 44with fastener-end 48 lies at an angle B of approximately 15° withrespect to axis 30'.

The radii R₁ and R₂ and the location of centers 32 and 43 are chosen sothat an interference fit of from about 0.004 to 0.015 inches andpreferably from about 0.008 to 0.012 inches is obtained at the surface36 of workpiece 18 when the fastener 20 is fully installed withinworkpiece 18. Similarly, the radii and center locations of circles 31and 41 are chosen so that an interference fit of from 0 to 0.010 inchesand preferably from 0.004 to 0.008 occurs between shank 22 and bore 26when the fastener is fully inserted into workpiece 18. This interferencefit is obtained by deformation of fastener or deformation of bothfastener and workpiece.

In FIG. 2, a completed structural joint is shown joining workpieces 18and 19 together by means of a continuous curved head rivet 20. Rivet 20has an upset or peened end with head 80 formed tightly against surface17 of workpiece 19. This structure may be easily formed by the method ofU.S. Pat. No. 3,874,070. Head 24 of rivet 20 is shown pressed intoworkpiece 18 so that surface 48 is flush with surface 36. In thisembodiment, the rivet 20 and the bore of the aperture through theworkpieces are coaxial on axes 30-30'. Forcible insertion of the rivet20 causes a certain amount of the material of rivet 20 to be displacedin a direction perpendicular to axes 30-30' as shown by the smallarrows. This interference, preferably in the form of a controlledlaminar interference pattern, causes residual stresses to be set up inboth the rivet and the workpiece material adjacent the bore, inducinghoop stresses which strengthen the workpiece structure. When an aluminumrivet is used with an aluminum workpiece, deformation of 0.004 to 0.005inches is typical for rivets having a shank diameter of about 0.2 to 0.4inches.

The rivet as shown in FIGS. 1 and 2, having a smooth exterior surface inthe form of a continuously curved head as shown, is conveniently pressedinto the countersink until the zone 56 resists further entrance of therivet 20 into the aperture through workpieces 18 and 19. Zone 56, beingan outwardly flared portion of head 24, provides sufficient resistanceto pull-through of the rivet 20 and is located so that, upon forcibleinsertion of rivet 20, the surfaces 48 and 36 are substantially coplanaras shown in FIG. 2. The portion of the head shown approximately at zone57 is contoured so that the portion adjacent surface 48 approachesperpendicularity with respect to surface 48. Forcing rivet 20 intoworkpiece 18 causes inward radial displacement of the rivet materialadjacent zone 57 and outwardly directed displacement of the adjacentworkpiece material. Hoop stresses are set up in the workpiece materialsurrounding the countersink and the rivet head. The resulting structureis substantially strengthened thereby.

In FIG. 3, a "captured" type of rivet and countersink structure isshown. In these Figures, the symbol R₁ is used to denote the radius ofthe upper portion of the generatrix describing the fastener head priorto insertion into the workpiece aperture, while R₂ is utilized to denotethe radius of the upper portion of the countersink. The fastener in FIG.3 is shown captured within the workpieces 68 and 69 and the center ofthe upper arc of the generatrix is shown positioned on the axis 61 belowsurface 66. A smaller circle 63 is shown forming the portion of thegeneratrix tangent to both the shank of the fastener and the upperportion of the generatrix. In FIG. 3, the centers of the generatricesfor both the countersink and the rivet head are shown coincident. As aresult, the interference between rivet 62 and workpiece 68 will besubstantially equal over the entire surface of the head of rivet 62.

Die 64 having a cup-shaped rivet-engaging surface is preferably used forthe captured type of rivet structure to encourage flow of the rivetmaterials at the periphery of the rivet to adequately fill thecountersink. The shape of the die shown causes the peripheral areas ofthe rivet to deform first, which in turn enhances the filling of theconcave cavity of the continuously curved countersink.

In FIG. 4, the center of the circle forming the upper portion of thegeneratrix of the workpiece aperture is positioned above the surface ofthe workpiece. In FIG. 5, the generatrix of the countersink is comprisedof a section of a hyperbola. Other smooth, curved segments such as asection of an ellipse may be used. The rivet used in this invention mayhave a substantial taper along its length so that an interference fitmay be conveniently formed along the entire length of the bore throughthe workpieces. Alternatively, the fastener, the bore or both may becylindrical.

In FIG. 6, the centers of circles utilized to form the upper portion ofthe generatrices of both the countersink and the rivet head arecoincident at the upper surface of the rivet head. In this embodiment,equivalent interference will be encountered between the rivet head 71and workpiece 70 along the entire surface of the countersink.

Other surfaces of rotation may be utilized to define the surfaces of thecountersink and rivet head. Clearly, elliptical sections, hyperbolicsections and other smooth, curved segments may be used as generatricesfor the fastener head and countersink, the prime requirement being thata smooth, curved surface is generated from the shank of the rivet to thetop of the rivet head and from the bore to the surface of the workpiece.

Other configurations of this invention include fasteners andcountersinks having the surfaces defined by rotation of curved linesabout axes non-coincident with the axis of fastener pin and countersink.

A substantially cylindrical slug 77, shown in FIG. 4, may be usedinstead of the preformed rivet blank discussed above. In all instancesthe excess rivet material above the surface, as is best seen in FIGS. 3and 6 may be removed to provide a smooth aerodynamic surface faired withthe adjacent workpiece surface.

In FIG. 6 a truncated conical tool is used as the driver forinstallation of the rivets. The truncated conical shape causes theperipheral surface areas of the rivet slug to fill the bore andcountersink areas smoothly and completely while cold working the slugmaterial and the adjacent workpiece.

The structure and process of this invention may advantageously be used,for example, in adhering the aerodynamic skin of an aircraft to thestructural elements, for example, stringers or ribs.

It will be appreciated that the preferred embodiment of this inventionhas been illustrated and described as a riveting process and structurein which a rivet is formed into an aperture through workpieces which hasa continuously curved countersink diverging outwardly from the bore ofthe aperture while presenting a smooth, substantially tangent juncturewith the bore to form a convex surface which then forms a smoothjuncture with another curved surface which is concave and which extendsto the surface of the workpiece. While the preferred embodiment usessurfaces of revolution generated by rotation of a curved line formed ofcircular segments about the axis of the aperture, other surfaces ofrevolution could equally well describe the contours of the countersink.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A process for rivetingworkpieces together comprising the steps of:forming an aperture throughsaid workpieces said aperture including a bore and having a countersinkadjacent a first surface of one of said workpieces, said countersinkcomprising a continuously curved surface extending from said firstsurface of said workpiece to said bore, said countersink having aconcave portion extending from said first surface toward said bore andhaving a convex portion extending from said concave portion to saidbore; placing a rivet blank to said bore and countersink; and upsettingsaid rivet blank into intimate contact with said countersink to form astructural fluid-tight joint.
 2. The process of claim 1 wherein saidcountersink has a contour described by a surface of rotation.
 3. Theprocess of claim 2 wherein said surface of rotation is formed byrotation of a generatrix comprising a continuously curved line about anaxis coincident with the axis of said aperture.
 4. The process of claim2 where said generatrix comprises segments of two circles one of saidtwo circles being tangent to said bore and to the other of said circles.5. The process of claim 1 wherein said rivet blank comprises a rivethaving a partially preformed head.
 6. The process of claim 1 whereinsaid rivet blank comprises a substantially cylindrical slug.
 7. Theprocess of claims 1 or 6 wherein both said rivet and the portions ofsaid workpieces adjacent said aperture are cold worked during saidupsetting step.